Process of preparing wood pulp



Patented July 13, 1943 2,324,230 PROCESS OF PREPARING WOOD PULP Fredrich Olsen, Soldon Feldman, and Lyle Melvin Sheldon, East Alton, Ill., assignors to The Cellulose ResearehCorporation, East Alton, Ill., a corporation of Delaware No Drawing.

Application January 30, 1939,

Serial No. 253,598

6 Claims.

This invention relates to the preparation of c'ellulosic pulp from wood.

An object of the invention is the production of wood cellulosic pulp suitable for use in the manufacture of cellulose acetate, propionate, butyrate, other esters, and other derivatives of cellulose capable of yielding clear, brilliant, solutions in acetic acid, acetone, or other suitable solvents.

We have now discovered an improvement over prior processes whereby purified wood cellulosic pulp of improved quality for the purposes named is produced more simply and at a material reduction in cost.

Our invention consists of a process for the substantially complete removal of ligneous impurities from wood cellulosic material, prior to subjecting the pulp to alkali or hypochlorite treatments, by a sulfite digestion controlled to provide uniform pulp of a bleachability of not more than about 15%, preferably less than cocrdinated with a uniform completely delignifying chlorination of the digested pulp with not less than an amount of chlorine, expressed in pounds per 100 pounds of pulp, equivalent to 20% of its bleach value. The chlorinated pulp is thereafter'treated by a light alkaline hypochlorite bleach, strong alkali under mercerizing conditions to remove residual coloring material and to impart enhanced reactivity such as to yield solutions 'of the resulting acetate of not more than 150 p. p. in. color and 50 p. p. m. haze.-

We have found that unless substantially all of these residual impurities responsive to chlorination processes are rendered soluble prior to the use of oxidizing treatments such as alkali and hypochlorite solutions, they tend to become fixed on the pulp by such alkaline steps and. are not removable therefrom without employing numerous expensive processing steps.

By substantially complete removal of impurities which are responsive to chlorination, we mean the removal of these impurities to an extent which has not heretofore been accomplished. We define the substantially complete removal of these impurities as being measured by a KMnO4 number bleachability of the pulp of 1% or less.

We have further found that in order to eifect this complete removal of impurities which are responsive to chlorination, as measured by a KMnOr number bleachability of the pulp of less than 1%, that it is essential to digest the wood to a low degree of bleachability. We have found the digestion has been carried to such an extent, all of the material of this type will not react with the chlorine in a single step. This is probably due to the fact that the chlorination reaction is a topochemical one and that the chlorinated products are not directly soluble in the chlorinating solution but must be removed subsequently. Thus when substantially more than an amount of chlorinatable material corresponding to a KMnO4 number bleachability of about 15% is present in a raw pulp after digestion, the chlorine reacts with the exposed chlorinatable material but is not able to subsequently penetrate through the layer of chlorinated material thus formed, to react with the remainder of the material of this type. For example, we have found that with a rawpulp having a bleachability of 25%, it is not possible to reduce the bleachability by chlorination below about 2.5%, regardless of the amount of chlorine added to the pulp or the time of reaction. However, if the KMnOa. number bleachability of the pulp, has

been reduced to about 15% or less, and preferably less than 10% by digestion, the bleach value may be reduced to 1% or less by the chlorination process described herein.

The essential feature of the chlorination process is the employment of not less than a certain minimum amount of chlorine based on the chlorine demand of the raw pulp digested to the extent described above. We have found that this minimum amount may be readily calculated and expressed in pounds of chlorine per 100 pounds of pulp as not less than 20% of the bleach value of the pulp. This amount is usually suflicient to provide a slight excess of chlorine at the end of the chlorination period. It is essential that this slight excess of chlorine be provided, since it constitutes the driving force which is necessary to force the chlorination reaction to completion and thus to reduce the KMnO4 number bleach value of the pulp to 1% or less.

Thus to summarize, in the process of our invention the wood is uniformly digested to a low bleachability, preferably less than 10%, in a manner which results in such a homogeneous distribution of the residual chlorinatable content 0! the pulp that chlorine is able to penetrate and react with all of the remaining material of this type. The chlorine compounds thus formed are removed in a subsequently applied dilute alkaline that in order to obtain a KMnO4 number bleachtreatment and the pulp thus rendered free of material soluble in bisulfite and in chlorine. For example, the most sensitive of the known tests do not indicate the presence of even a trace of lignin.

Thus, by the coordination of the uniform digestion and chlorination, the completion of the purification comprises merely the removal of a very small residue of oxidizable material, remainan apparently fixed, small residue of impurities in the second, both treatments designed to avoid chemical degradation of the cellulose. This system requires the addition of a much larger portion of the total amount of chlorine'in the -first agent to the non-cellulosic material, under conditions whereby the reactions of the several reagents employed will be highly preferential with respect to the specific impurities. The raw pulp is first thoroughly washed and relieved of all impurities notchemically bound and then subjected to a two stage chlorine treatment. In the first stage, the chlorine is used in thegaseous state. to form a chlorine water solution. The

stage than is customary in the typical prior art practices.

total chlorine employed is added in the first stage; with the very low bleachability raw pulps preferred in our process, for example, pulpsof less than 5% bleach values, at least 50% is used in the first stage. By this apportionment of a very large percentage of the total chlorineto the first stage, the consumption in the hypochlorite stage is substantially uniform regardless of the raw pulp bleaehability, being approximately only 0.3%-0.4% of bleaching powder on the .weight of the pulp.

By our process the fibers are brought to a nearly homogeneous state by a digestion designed tosubstantially eliminate degradation. In this step the fibers are uniformly and mildly treated under an appropriate time-temperature schedule until only an amount of lignin remains associated therewith which is capable of complete removal by chlorination. Since the action of chlorine is a topochemical one, it is necessary to remove by .3

digestion a sufiicient amount of lignin from all portions of every fiber including the fiber tips, such that all of the residual lignin will be readily accessible to the chlorine. This may be effected by a thorough preimpregnation of the wood form of chips, or other suitable, relatively'fiat subdivisions, with a bisulfite delignifying solution at a temperature below the normal reaction temperature of the reagent. The bisulfite solution preferably comprises a combined SO: content of at least 1.1%. The impregnated wood pieces are then'digested under essentially mild conditions of time and temperature to 'yield a pulp of low bleachability, preferable less than or even less than 5%, as will be hereafter described in typical examples of the operation of the process. It is desirable to arrange the timetemperature schedule to give a preferential action of the reagent on the ligneous material and to effect a nearly complete removal of bisulflte reactive material. At the same time the depolymerizing action of the solution is preferably held within such a range whereby the viscosity of the For example, with raw pulps of 10% to bleachabilities, 80%-90% or more of the cellulosic material is maintained above at least.v

10 centipoises.

This condition of the raw pulp is ideal for the subsequently applied purification to eliminate the residual ligneous and other non-cellulosic material. The chlorination step is thus able to com;-

plete the delignification of the wood so that all ,process is directed toward the complete removal 'of non-cellulosic impurities from the individual amount employed is substantially in excess of that which will exhaust in about 30 minutes; we have found that the effective amount can be readily calculated and expressed in pounds per 100 pounds of pulp as not less than 20% of the bleach value of the raw pulp. The gas is added to the pulp slurry as rapidly as possible and the treatment carried out under conditions designed to effect complete chlorination, and avoid oxidation of the non-cellulosic material. The effect of the chlorination is to purify the raw pulp to a uniform standard measurable by a bleach value of about 1% or less, irrespective of its initial bleachability in the raw pulp state. This standardization of the pulp quality at this point of the process permits a simplified, uniform further purification of the pulp to the desired high quality in subsequently applied treatments.

After the first stage chlorine treatment, the pulp thus contains a small, nearly invariable amount of oxidizable material irrespective of the raw pulp bleachability. In the second stage, the chlorine is used in the form of hypochlorite.

In some cases it may be desirable to carry out the chlorination in two stages, particularly where especially high pulp viscosity is required. In such cases an amount of chlorine equal to about 12% to 15% of the bleachability of the pulp is used in the first treatment. The pulp is then neutralized with alkali, washed, and again treated with an amount of chlorine equal to 6% to 8% of the raw pulp bleachability. Such double chlorination requires a somewhat smaller total amount of chlorine, and results in less depolymerization of the cellulose than is incident to a single stage chlorination.

cient amount of alkali may be added to the chlorinated slurry to provide a relatively dilute concentration and the treatment carried out at elevated temperature until the chlorinated comwe pounds are removed. The pulp may next be given a mild alkaline hypochlorite bleach to remove oxidizable impurities and to reduce the viscosity to the desired value. The amount of reagent employed is pref erably in excess of that which is necessaryto effect the desired decolorizing" and the .bleach is terminated upon the attainment of the desired viscosity and before exhaustion of the bleaching fibers in the wood with meticulous care in the preservation of the cellulosic component of each fiber, by providing complete accessibility of rereagents in the bleach liquor,

The bleached pulp may be given further treatments for the purpose of imparting higher reactivity and to eliminate remaining traces of substances which tend to impart color to the solvent solutions of acetate and like derivatives of the pulp. For example, the pulp may be treated in sodium hydroxide solution under mercerizing conditions. The properties of the mercerized pulp are especially afiected by the procedure followed in removing the alkali from the cellulose at the completion of the mercerizing treatment. Usually there is a strong tendency to the formation of neps which are very tightly bound bundles of fibers, as the removal of the alkali proceeds.

The time employed for the removal should be as short as possible; with suitable apparatus this can be accomplished in less than 1 hour and it is desirable to complete the washing in 15 minparted to said solutions by re-use until the organic content has been built up to the range mentioned and is maintained at a fixed equilibrium. Such liquors in the range of 5% concentration, up to and including mercerizing strength, may be used without detriment to the pulp properties provided certain precautions are observed in removing the liquor from the pulp cake after the completion of the treatment.

The most important step in this method of removing the treating solution is to add a small displacement wash of fresh mercerizing strength caustic soda solution equal in volume to about 30% of the amount of liquor in the pulp cake at about 13% consistency. The fresh solution acts as a liquid piston and effectively forces the reaction liquor out of the cake with substantially no precipitation of the organic matter when the remaining caustic in the pulp is thereafter removed by means of. displacement wash with fresh water. A counter current washing system of washing may also he sometimes advantageously employed, if a somewhat larger proportion of the fresh caustic displacement wash is employed.

By using this system of washing, formation of neps is completely avoided and the highly desirable reactivity and other advantageous properties of. the mercerized pulp are fully maintained. It is to be understood, however, that we do not herein claim the use of alkali solutions high in organic content and the washing process described. I

The pulp after mercerization or after the nonmercerizing caustic treatment may be given a further treatment, particularly where especially low color in the solvent solutions of the acetate is desired. In such cases, a dilute water slurry of the pulp is given a light chlorination, for example, 0.1%-0.2% chlorine based on the weight of the pulp, for 5 to 15 minutes at room temperature and thereafter washed until neutral. In lieu of the chlorine treatment, the pulp may be treated in a dilute solution 'of sulfurous acid, for

example, 0.01% concentration, for about 30 minutes at room temperature or if desired it may be given both a light chlorination and sulfurous.

acid wash.

By providing a digested pulp in which the residual ligneous and other impurities associated with each fiber is accessible and capable of complete reaction and removal, thepurification treatments described are able to reach and react with the residual impurities existing in the most inaccessible fiber tips, so that each fiber in the mass of mercerized material and every portion of each fiber is completely cleared of those impurities which ordinarily interfere with commercial use of wood cellulosic pulp in the production of acetate and other esters difiicult to produce in a condition of acceptable purity. The individual fibers of the mercerized pulp are of heretofore unattained chemical and structural uniformity and are completely and immediately responsive throughout their entire length to acetylating or other reagents, with the result that such reactions proceed smoothly and rapidly to completion leaving no unreacted particles, particularly fiber tips. The solutions of acetate in appropriate solvents are characterized by substantially lower values of color or haze.

In order to particularly describe the present invention, there follows the description of typical embodiments. It will be understood that the various features of the inventions set forth in connection with these examples are by way of illustration only, and may be considerably varied within the scope of the present invention which is not limited thereto.

EXAMPLE A Black gum wood, selected to insure freedom from knots and decay, was chipped to the standard inch size usually used in the pulping industry. The chips, with a moisture content of 13.1%,

were charged into the digester and the air removed from the chips by introducing steam at At the end of the steaming period, when substantially all of the air had been removed from the chip and replaced by steam, the bottom vent was closed and the steam supply to the top of the digester cut off. Calcium base acid sulfite liquor, having a total S02 content of 5.99%, free S02 of 4.81%, and combined S02 of 1.18%, preheated to a temperature of C. was then pumped into the digester. The ratio of liquor added to oven-dry wood was 6.0 to 1 by weight. The introduction of this cooking acid at 90 C. caused the steam present in the interstices of the wood to condense and draw the cooking liquor into the wood chips, and thus effectively impregnate the latter with the cooking acid. The temperature of the wood chips and of the acid in the digester was then raised uniformly to 133 c.

in 2.5 hours bymeans of indirect steam and maintained at 135 C. for 5 hours. At the end of this period, the steam supply to the digester was shut off and the temperature allowed to drop to 120 C. This required 6.5 hours. The temperature was then maintained at 120123 C. for 11 hours longer. The pressure with n the digester was maintained at a maximum of 75 pounds per square inch during the entire cook.

The digester employed was equipped with a false bottom, so that at the termination of the cooking period, which required a total of 25.5 hours, the spent cooking liquor was blown from the digester through an outlet at the bottom. Th thoroughlybooked pulp wh ch remained in the digester was then washed completely with hot water at C. while still in the digester. until the wash water was completely free of acid and coloring matter. The washed pulp was then d scharged into a classifier where defibering of the cooked chips was accomplished by violent agitation and the defibered pulp classified for the removal of fines. As a precaution, the defibered and classified pulp was then screened through a plate screen having 0.010 inch slots for the removal of any undefibered chips, knots, or other impurities. Howevenno such material was found. The net yield resulting from the digestion. classification, and screening processes was 44.4% based on the over-dry wood. Analyses of the raw pulp at this point showed the following values:

Cuprammonium viscosity (0.6% concentration) s ctp.. 11.5 Bleachability (determined by the permanganate number method, expressed in terms of standard bleaching powder containing 35% available chlorine) -per cent..- 1.6

The classified pulp was then subjected to the following sequence of purification treatments:

(a) Chlorination.

(5) Treatment with boiling dilute causticv soda solution.

(c) Sodium hypochlorite bleach.

(d) Mercerization.

(e) Final washwith dilute sulfurous acid.

Chlorination The pulp was treated at 3% consistency and at a temperature of 26 C. with 0.42% chlorine based on the oven-dry pulp. This amount of chlorine represents 26% of the bleach value of the raw pulp. The chlorine gas was added directly'to the "pulp slurry at a uniform rate over a period of minutes and the reaction was allowed to proceed for an additional period of 35 minutes.

The slurry was then neutralized to a pH of 8.5-9.5 by NaOH added in the form of a 2% solution, Immediately after neutralization, the spent chlorination liquor. was drained from the pulp and the latter 'washed with cold water. At this point the pulp had a bleach value of 0.7%.

Treatment with boiling dilute caustic soda solution The washed chlorinated pulp was then boiled at 3% consistency in a 1% NaOH solution. The temperature of the slurry was raised to 100 C. in

2.5 hours in a jacketed tank and then maintained Sodium hypochlorite bleach The pulp was then bleached with sodium hypochlorite under the following conditions:

Consistency percent 3 Percent bleach based on oven-dry pulp percent 1.5 Temperature -1 C 26 Time urs 3 pH during bleaching 9.0-9.5

At the end of the bleaching treatment the pulp. was thoroughly washed with cold water.

- M ercerization The bleached pulp was then mercerized by treatment at 3% consistency with 13.7% NaOH at 21 C. for 15 minutes. this period the pulp was washed with cold water in a tank equipped with a false bottom and hav- At the conclusion of I ing facilities for agitation, until thepulp was entirely free of caustic.

Final wash. with dilute sulfurous acid The washed mercerized pulp was treated for 30 minutes at a consistency of 3% and a temperature of 21 C. with a 0.01% solution of S02. At the end of 30 minutes the reaction liquor was drained from the pulp and the latter thoroughly washed with cold water.

In the following Example B, is described the pulping of wood to a somewhat higher bleachability and the subsequent purification thereof.

Exmn B Black gum wood, selected to insure freedom of knots and decay was chipped to the standard /4 inch size usually used in the pulpingindustry. Seven hundred eighteen pounds (oven-dry basis) of chips were charged into a rotary digester 12 ft. in length and 5 ft. in diameter. When charged into the digester, themoisture content of the chips was 13.7%. i

The wood charge was then heated to a temperature of 90 C. and maintained at this temper ature level for a period of one hour by means of steam added directly into the digester. At the end of the hour period, the pressure within the digester, which amounted to approximately 12 lb. per sq. in., was relieved through the vent lines. The condensate formed during th hour period was allowed to drain from the digester.

Five hundred ninety seven gallons of calcium base acid sulflte liquor, having a total SO: content of 5.38%, free S0: or 4.04% and a combined S0: of 1.34%, preheated to a temperature of 90-94 C., was then added to the digester. including the moisture held by the wood at the completion of the steaming period and the volume of cooking liquor added, the ratio of liquor to oven-dry wood, expressed as pounds per 100. pounds, was 7.5 to 1. The temperatureof the dlgester and its contents was heated at a uniform rate from 90 C. to 122 C. in a period of 3 hours. Upon reaching the temperature of 122 0;, this level was maintained within 1 C. for a period of 2 hours. At the end of the 2 hour period at which point the cook was terminated. To re-v capitulate; the cooking schedulewas as follows:

Time Temperature Pressure C. to 122 0..." 28M At 122 58#. o 9% to 77#. 3% hours Total 10% hours. No pressure releases were practiced during the course of the cook. At the end of the cooking period, the pressure was reduced to 18# per sq. in. over a period of 40 minutes.

The digester employed was equipped with a false bottom which allowed a major portion of the washing to be completed in the digester. Cold soft water (20 C.), delivered at a pressure of 45#50# per sq. in., was employed for all washes. When the S02 content of the wash water discharging from the digester had been reduced to 0.1% or less, the pulp was discharged into a dump tank, where it was subjected to violent agitation to defiber the cooked chips. Any acid and water soluble coloring matter remaining in the defibered pulp was then removed by means of additional water washes.

The washed pulp was next classified for removal of fines and screened through a plate screen having 0.010 inch slots for the removal of any improperly cooked chips, knots, etc. Analyses of the raw pulp at this point showed the following values:

Cuprammonium viscosity, 0.6% conc ctp 32.9

Bleachability (measured by the amount of material oxidizable by potassium permanganate in presence of acid, expressed in terms of standard bleaching powder contaning 35% available chlorine) percent 10.6

The classified pulp was then subjected to further purificat on treatments which comprised the following steps:

(a) Chlorination and neutralization.

(b) Dilute caustic soda pressure treatment.

() Sodium hypochlorite bleach.

(d) Mercerization.

(6) Reducing bleach with sulfurous acid.

The detailed conditions of these purification steps are outlined in the following:

(A) Chlorination to a pH of 8.5-9.5 by NaO-H added in the form of a 2% solution. At the time the caustic used for neutralization was added, there was some chlorine remaining in the liquor which had not reacted with the pulp. The hypochlorite thus formed by the chlorine and caustic was exhausted 9 minutes after the addition of alkali. The spent chlorination liquor was then drained from the pulp and the soluble coloring materials and reaction residues were removed by means of a water displacement Wash. The following analyses show the changes which were effected bythe chlorination step:

- Cuprammonium Bleachability vise" 05% Comm Before After Before After Per cent Per cent Ctp. Ctp.

(B) Dilute caustic soda pressure treatment The washed pulp from the preceding chlorination step was then subjected to a treatment with 1% NaOH solution at a consistency of 3.0%. The temperature of the slurry was raised to 116 C.

during a period of 3.5 hours and the reaction was allowed to proceed for an additional hour at 1l4.5%-116 C. Prior to draining the reaction liquor from th pulp, the slurry was cooled to I C. After draining the free liquor; the reaction liquor held by the pulp and the soluble coloring materials were removed by displacement washes with fresh water. Analyses at this stage of the purification showed the cuprammonium viscosity to be 22.7 ctp.

(C'), Sodium hypochlorite bleach A small amount of oxidizable materials still remained in the pulp after the preceding chlorination and neutralization and caustic pressure boil. This material was removed with a sodium hypochlorite bleach conducted under the following conditions:

Consistency 2.7%.

Per cent bleach based on oven dry pulp 1.39 Average temperature .C 24 Time hrs 3 pH during bleaching 9.5-9.0

The average concentration of bleaching powder in the reaction liquor during the course of the treatment was 0.29 gram per liter. At the start of the treatment the concentration of bleach was 0.39 gm. per liter and at the end,

' the viscosity of the cellulose was 22.9 ctp.

(D) M ercerization Th pulp was then dewatered to a consistency of 9.0% and immersed in a suflicient volume of 20.5% NaOH solution to give a calculated equilibrium concentration of'l3.7% NaOH and a consistency of 2.5%. The pulp wa thoroughly dispersed in the caustic soda solution and the reaction was allowed to proceed for 1.9 hrs. at a temperature of 25-26 C. Filtration of a portion of the reaction liquor drained from the pulp showed a caustic soda concentration of 13.4% by weight. Removal of the caustic soda from the pulp was efiected by washing with water-in a tank equipped with a false bottom and having facilities for agitation. The entire washing cycle covered a period of approximately 3.0 hours. Some additional classification resulted from the washing necessary to free the pulp of the mercerizing alkali.

(E) Reducing bleach treatment Following the mercerization the pulp was treated for 23 minutes at a consistency of 2.4% and a temperature of 20-22" C. with a S02 solution of 0.01% concentration. The pH of the reducing bleach liquor was 3.2. Removal of the S02 from the pulp was accomplished by draining the reaction liquor and then washing until the pH had been raised to 7.0-7.3 with fresh water.

After the completion of the purification treatments described, this product had th following analyses:

Alpha celliilose per cent 98.5

, 0.006 inch Slots.

After drying, the pulp yielded an acetic acid solution of acetate having a 130 p. p. in. color, and 30 p. p. m. haze.

EXAMPLE C Black gum wood is treated as describedeither in Example A or Example B through the step of bleaching with hypochlorite solutions. The pulp is then subjected to a sodium hydroxide solution of 6.5% concentration at a consistency of 3% for In carrying out the process, considerable variation within prescribed limits may be made from the practice described in the foregoing examples.

It is preferable to use wood which retains the greater part of its natural moisture, which in some cases may be in excess of 50%. The moisture content is preferably held between 40% and 60% and if variations outside these limits occur, corrections in the volume and composition of the digesting liquor added should be made to compensate for the water in the wood. Other bisulfite treating solutions, such as sodium and ammonium bisulfite, may be used as well as calcium bisulfite. In any case, it is desirable that the combined SO: content be at least 1.1%.

The impregnation technique described in the foregoing examples may be supplemented or replaced by other methods of impregnation. For instance, in addition to the improvement in the rate of penetration of cooking liquor due to the removal of a substantial portion of the air contained within the wood chips by the steaming process, benefit may result from a series of pressure releases made at the proper temperature level during the actual digestion cycle. Release of pressure, when the temperature of the chips and the liquid which they contain is above the level at which boiling at atmospheric pressure will take place, will cause ebullition and as a resuit additional amounts of air will be flushed out of the chips. The temperature at which ebullition takes place upon opening the relief line will depend upon the height of the liquor and chip charge in the digester. When the well known type tall stationary digesters are used, the hydrostatic pressure resulting from the weight of the chips ,and liquor will materially increase the temperature at which ebullition of the cooking liquor at the bottom of the digester will take place upon opening the relief line. Thus the temperature range of 103-106 C., which is for a small digester in which the height of the liquor column is from 3-5 feet, will have to be raised to a higher level in the usual stationary digestion equipment, depending upon the height of the charge of chips and liquor.

We have found that the characteristics of the pulp can be varied over a wide range by gas releases made early in the cook, for example, during the sulfonation'period at which the temperature is maintained at about 122 C. and also before this temperature level is reached. Hence in order to produce pulp of the highest uniformity, I do not practice SO: releases, other than for the express purpose of aiding the impregnation by the removal of air, until substantially complete sulfonation of the non-cellulosic components of the wood is effected. This reaction takes place at a very desirable rate at the 122 C. temperature level. After the ligneous material is properly sulfonated, wide variations in gas relief can be made with substantially little variations in the characteristics in the end pulp product. However, to duplicate pulp characteristics, close adherence to the predetermined temperature schedule during the later stages of the cock is necessary, since the rate of hydrolysis of the sulfonated ligneous material is readily affected by variations in temperature.

The impregnation of the chips may be carried out in other ways than those previously described, for instance, by submerging the chips in water in the digester, boiling for 15 to 30 minutes and then displacing the excess water through an opening at the bottom of the digester by means of superheated steam added above the liquor level. In order to keep the amount of water absorbed by the wood to a minimum, it is important to keep the water used for impregnation purposes boiling prior to and during its displacement by'the steam. If the temperature is allowed to drop slightly before boiling, the amount of liquor absorbed by the wood will be greatly increased, thus requiring the use of much more concentrated digestion liquors to compensate for the excess water present. By this practice, the steam with which the digester is filled condenses very rapidly as soon as the cooking liquor is added, creating a vacuum within the interstices of the wood and also within the digester. By this procedure the chips are free of air and impregnated with the treating solution without first becoming saturated with water. Furthermore the chips are impregnated, not only with S02, which clifiuses into them more rapidly than the base, but with the base as well, since the liquor is drawn into the chips in presumably unaltered form.

If desired, the preimpregnation 0f the wood with water may be omitted and the chips directly boiled in sodium-sulfite or ammonium bisulflte solutions containing substantially no free S02. In such cases, drainage of the excess liquor is unnecessary. When the wood is thus directly impregnated with solutions containing practically no free S02, the requisite amount of S0: is added directly to the liquorin which the chips are submerged after the impregnation has been accomplished.

Where the facilities permit,- it is preferable to wash the digested pulp in hot water, for instance, at or above C. and even as high as C., since some of the impurities in the pulp are more soluble in water at elevated temperature than at normal temperatures.

Classification of the pulp may be practiced at any convenient point in the process. It is preferable to classify the raw pulp since this results in a reduction in the amount, of reagents necessary for the subsequent purification because the burden on' the later steps is reduced by this tus consists of a rubber lined tank, equipped with agitation of Allegheny metal construction and an Allegheny mesh screen cover. The pulp, as a temperature.

0.25% to 1% slurry in purified water, is fed continuously into the tank, the level being maintained at such a height that the agitation throws the slurry against the screen cover and imparts a pulsating action thereto. The continuous agitation avoids matting of the pulp on the screen and the pulsating action imparted to the slurry discharges the fines through the screen while the water in the slurry washes the fines from the upper portion ofthe screen, which is installed at a slight angle for this purpose.

The long fibered fraction of the pulp is discharged into an appropriate vessel while the overflow of fines is collected in a separate tank. The long fibered fraction is then given the requisite pu ification and the fine material thus accumulated may be used in admixture with paper pulp where the purity requirement is not so high, or for other purposes.

In carrying out the chlorination the gas is added to the pulp slurry as rapidly as possible, consistent with thorough distribution of the reagent to every particle of the pulp. Usually this will require from 5 to 30 minutes. The complete reaction with the lignin and other chlorinatable substances is usually accomplished in from to 30 minutes; prolonged treatment is unnecessary and tends to reduce the viscosity of the pulp.

The amount of chlorine employed should not be less than that amount calculated as of' the bleach value of the pulp. Actually to of the bleach value may sometimes be required in order to effect the necessary degree of chlorination. As the amount of chlorine is increased, evidence of lowering of viscosity is noted, hence the chlorine dosage should be kept to the minimum, consistent with reduction of the bleach value after chlorination to 1% or less. Upon neutralization of the chlorinated slurry by the addition of alkali, the pulp may be treated if desired in the hypochlorite thus formed but no noteworthy benefits have been found to accrue by such treatment other than a reduction in the amount of bleach otherwise required in the hypochlorite bleach step of the process.

If desired, the washing'step between the chlorination and the dilute caustic treatment may be omitted without detriment to the pulp properties. In such cases the chlorination is terminated by adding a sufiicient amount of alkali, for example,

caustic soda, to form a concentration of between and 3%. The mass is then heated to atmospheric or greater than atmospheric pressure and treated for about 30 minutes at this elevated The removal of chlorinated lignin products and other impurities which influence the color and haze properties of the acetate that can be produced'may also be effected with milk of lime solutions at about 100 C. A milk of lime treatment containing 7.2% reagent based on the pulp at 10% consistency for 6 hours at 100 C. will give satisfactory results. It should be pointed out that the analytical values of the finished pulp shown in the example are dependent on the use of NaOH in the dilute alkali boiling treatment following chlorination. With the use of milk of lime solutions, the soda soluble content of the finished pulp will be somewhat higher than the 2% to 2.5%, indicated in the example. Likewise,-

with caustic soda solutions, the soda soluble value in the finished pulp will vary inversely with the chemical to pulp ratio. In general, the lowest acetate dope color and haze values are attained by regulating the conditions of the dilute alkali treatment to reduce the soda soluble content of the purified pulp to less than 3 Upon the completion of the removal of the chlorinated compounds, the pulp will have a bleach value of about 1% or less. At this stage of its purification it is capable of conversion into grain free tri-acetate yielding solutions in suitable solvent having a haze of about 50 ppm. suitable for use in acetate products where very low color is not essential.

The pH of the pulp slurry coming from dilute alkaline boil is important and must be closely controlled since it provides a means for adjusting the pH of the hypochlorite bleach in its initial stages to the desired level. If the pH of the pulp slurry, prior to the addition of the hypochlorite stock solution, is between 7 and 8, the alkali contained in the bleach liquor to maintain stability Will bring the pH of the pulp slurry within the range of 8.0 and 9.5 when the hypochlorite solution is added. If the pH of the slurry prior to addition of the bleach liquor is outside these.

limits, we have found it desirable to make adjustments by the addition of either acid or alkali as required.

In the bleaching stage of the process calcium hypochlorite may be used, but sodium hypochlorite is preferable. The amount of hypochlorite should be substantially in excess of the amount actually consumed in the reaction. The excess is used for two reasons: to insure complete decolorizing and to provide a convenient means of controlling the viscosity of the pulp to approximately the value desired in the completely purified pulp product. The deligniiying and bleaching action is substantially completed in the early stage of the treatment during which period'the viscosity of the pulp decreases at a relatively rapid rate; thereafter the rate becomes relatively slow.

This feature provides a convenient means of accurately determining the proper time of treatment to give the desired pulp viscosity. During the latter portion of the bleach treatment, when the rate of viscosity reduction is slow, an actual determination is made and from the relation of this viscosity point to a standard viscosity reduction curve, the additional time of treatment needed to attain the desired pulp value can be obtained by extrapolation with an accuracy of 2 ctp. or less.

It is important to carry out the hypochlorite bleaching under alkalin conditions to avoid unnecessary degradation of the cellulosic component of the pulp. Sufficient alkali such as caustic soda is consequently added to the bleach liquor to tration; best results, however, are attained by.

13% to 15% concentration. The consistency may be from 4% to 12%, depending upon the facilities available for agitation; higher consistencies are preferred because of the resulting economy of reagent. The time of treatment may vary from 15 minutes to 4 hours. The equipment used therefore, forwashing the alkali solution from the mercerized pulp should be designed to effect the removal in the shortest possible time, and

- with a minimum exposure to air of the cellulose.

Vigorous agitation, of course, will accentuate the adverse eifect of a given time of exposure upon the above mentioned properties of the cellulose.

The vacuum filter type washer has been found to be very satisfactory for removing mercerizing .caustic soda from wood pulp. This type of "equipment will, in general, effect the most rapid and complete recovery of reagent possible and at the same time maintaining the concentration of alkali in the various wash liquor fractions at a high level.

The purified pulp is suitable for conversion into cellulose esters, ethers, and other derivatives but is especially adapted to the preparation of cellulose acetate, distinguished for the clarity,

and brilliancy of solutions in suitable solvent. For example, the acetone solutions of acetate have a color of less than 150 p. p. m. and a haze of not more than 50 p. p. m., these values bein frequently less than 50 p. p. m. each.

In the foregoing specification and following scribed by H. F. Lewis in Technical association papers, series XVII, #1, 436 (1934), described in copending application of Lyle Sheldon et al., S. N. 81,844, now Patent No. 2,185,776, filed May 26, 1936, pages 11-13.

Bleachability of cellulose, as used herein, is a measure of the materials oxidizable by potassium permanganate in the presence of an acid under specific conditions, and is expressed in terms of standard bleaching powder containing available chlorine. A detailed description of the method, including the conversion table for expressing the permanganate number in terms of per cent bleaching powder of 35% available chlorine, was published by T. A. P. P. 1., series XVII, #1, 146 (1934) Permanganate number of pulp," by R. N. Wiles.

Cuprammonium viscosity, as used herein. means the viscosity number or value obtained according to the following method: The cuprammonium solution was prepared by the action of air on electrolytic copper in the presence of strong ammonia water. The copper concentration of the solutions employed for viscosity determinations was 30:2 g. per liter and the ammonia content was 165 g.:2 g. per liter. The concentration of cellulose employed was 0.6 g. (oven dry basis) per 100 cc. of cuprammonium solution. The cellulose sample for this determination was dried at 70 C. to 4% moisture content. After weighingout 0.6 g. (oven dry basis), the sample was moistened, squeezed to a uniform weight of 2 g. and then dispersed in cuprammonium solution in an atmosphere of hydrogen from which oxygen had been completely removed. The viscosity measurements were made at 25 C. with a modified Ostwald pipet, constructed according to the specifications of the American Chemical Society Committee on the Viscosity of Cellulose (Journal of Industrial &-Engineering Chemistry, I, #49; 1929). The time of flow in secondsewas converted to centipoises on the basis of the calibration of the pipet with oils of known viscosity in centlpoises obtained from the United States Bureau of Standards.

Color and. haze. The measurements referred to herein of the color and haze of the acetic acid and acetone solutions of the acetate were made by comparisons with standards of known color and turbidity expressed in parts per million. The standards are those used for water analysis recommended by the American Public Health Association and consist of platinum cobalt (for color determinations) and fullers earth (for turbidity comparisons). With the materials recommended by the American Public Health Associatiorna series of standard solutions ranging from 10 p. p. m. to 200 p. p. m. inclusive at intervals of 10 p. p. m. were prepared for both color and haze. cc. of each of these solutions were hermetically sealed in an 8-ounce. wide-mouth, glass-stoppered bottle having an lntemal diameter of 2 inches. The acetate dope samplesto be measured were prepared in bottles identical with those employed in making the color and haze standards and compared under a controlled source of light, first with the color standards until the particular solution of each which most nearly corresponded to the dope under comparison was found, and then with the haze standards in conjunction with the most appropriate color standard. The measurements of the color and turbidity of the acetate dope are made exactly 1 hour after the dope has been killed by the addition of the dilute acetic acid. At the time of measi1rement,the dope must be absolutely free from air bubbles and at a temperature of 25 C., and the quantity of dope under comparison in the bottle must not be less than 100 cc. It has been found that color and haze ratings by this method can b made to an accuracy of approximately 10 p. p. m.

We claim:

1. The process of preparing wood pulp comprising the steps of digesting wood with a bisulfi'te solution by uniformly applying the said chemical and heat to each of the woody subdivisions until a pulp is formed with all the fibers having a. bleachability of less than about 15% and terminating the said digestion before the cellulose viscosity has been reduced to less than the order of 10 centipoises, forming a water'slurry of said pulp and chlorinating said pulp by uniformly and rapidly adding to said water slurry at room temperature an amount of gaseous chlorine expressed in pounds per 100 pounds of pulp equal to not less than 20% of its bleachability and causing said chlorine to penetrate and react with the chlorinatable content of the pulp prior to any substantial oxidation of the cellulosic components of said pulp, continuing said treatment until the bleach value of said pulp is reduced to not greater than about 1% and terminating said treatment before exhaustion of said chlorine.

2. The process of preparing wood pulp comprising the steps of digesting wood with a bisulfite solution by uniformly applyi g the said chemical and heat to each of the woody subdivisions until a pulp is formed with all the fibers having a bleachability of less than about 15% and terminating the said digestion before the cellulose viscosity has been reduced to less than the order of 10 centipoises, forming a water slurry of said pulp and chlorinating said pu p by adding to said water slurry at room temperature an amount of gaseous chlorine expressed in pounds per 100 pounds of pulp equal to not less than 20% of its bleachability and causing said chlorine to penetrate and react with the chlorinatable content of the pulp, continuing said treatment until the bleach value of said pulp is reduced to not greater than about 1% and terminating said treatment before exhaustion of said chlorine, treating said chlorinated pulp with a dilute solution of sodium hydroxide until the chlorinated compounds have been dissolved, treating said pulp in an excess of an alkaline hypochlorite solution, terminating said treatment when said pulp has been decolorized and the desired cuprammonium viscosity attained and before exhaustion of the reagent in said hypochlorite solution and thereafter treating for at least 15 minutes in a solution of sodium hydroxide at 13% to 15% concentration at room temperature and rapidly washing said pulp until free of reagent. I

3. The process of preparing wood pulp comprising the steps of digesting wood with an acid bisulfite solution by uniformly applying the said chemical and heat to each of the woody subdivisions until a pulp is formed having a bleachability oi less than about 5%, terminating the said digestion before the cellulose viscosity has been reduced to less than the order of centipoises, forming a water slurry of said pulp and chlorinating said pulp by uniformly and rapidly adding to said water slurry at room temperature an amount of gaseous chlorine expressed in pounds per 100 pounds of pulp equal to not less 5% and a viscosity not less than about 10 centipoises, forming a water slurry of said pulp and chlorinating said pulp by uniformly and rapidly adding to said water slurry at room temperature an amount of gaseous chlorine expressed in pounds per pounds of pulp equal to not less than 20% of its bleachability and causing said chlorine to penetrate and react with the.

chlorinatable content of the pulp prior to any substantial oxidation of the cellulosic components of said pulp, continuing said treatment until the bleach value of said pulp is reducedv to not greater than about 1% and terminating said treatment before exhaustion of said chlorine. 5. The process a set forth in claim 4 in which the chlorinated pulp is subjected to an alkaline hypochlorite bleaching treatment and thereafter to a treatment with caustic soda solution, each of said treatments being succeeded by washing.

6. The process of preparing wood pulp which comprises providing unbleached sulfite wood pulp having a bleachability less than about 15% and a viscosity not less than about 10 ctps., forming a water slurry of said pulp and uniformly and rapidly adding to said water slurry at room temperature an amount of gaseous chlorine expressed in pounds per 100 pounds of pulp equal to not less than 20% of its bleachability and causing said chlorine to penetrate and react with the chlorinatable content of the pulp prior to any substantial oxidation of the cellulosic component of said pulp, continuing said treatment until the bleach value ofsaid pulp is reducedto not greater than about 1%, terminating said treatment before exhaustion of said chlorine and thereafter subjecting the chlorinated pulp to an alkaline hypochlorite bleaching treatment and to a treatment with caustic sodasolution, each of said treatments being succeeded by washing.

FREDRICH OLSEN.

SOLDON FELDMAN.

LYLE MELVIN SHELDON. 

